Friction can be detrimental to the performance and operation of mechanical devices. An instrument (such as a medical device) that includes a movable actuator element (such as a pull cable or push rod) to operate an end effector (such as a grasper or scissor) of the instrument may experience considerable friction forces detrimental to its performance and operation. Controlling both static and dynamic friction is important. For example, negative slopes on the force vs. velocity curve (such as caused by static friction) are undesirable since they may lead to jerky stick-slip motion.
Also, sliding friction that results from the actuator element sliding over (or otherwise having physical contact with) other surfaces as it moves to actuate the end effector can result in significant loss of force transmission which reduces the available force to the end effector, thus adversely affecting its use. In particular, such force transmission loss means that more force must be exerted at an input end of the instrument (either manually for hand operated instruments or by an actuator that actuates the actuator element) to generate a desired force at the end effector. Thus, it becomes harder to exert the desired force to grasp or cut something using the end effector.
Accuracy of the force transmission is also important. As an example, assume that for continuous cable motion, only half the force applied to the proximal end of a cable reaches the distal end. In many applications, most of the cable motion will not be continuous or steady state. The cable motion will be intermittent, back and forth in both directions. As a result, if the applied force is known only at the proximal end, the force at the distal end may be any value between one half the proximal force to two times the proximal force. Thus, if the transmitted force is being used to tension a suture, the suture may be anywhere between only half as tight as intended and twice as tight as intended.
Force transmission loss and force transmission accuracy are of particular concern for flexible instruments (such as endoluminal devices used for performing medical procedures) since the available force to actuate or otherwise apply at the end effector decreases exponentially with the coefficient of friction and the total bend angle that the actuator element travels around. When actuator elements such as control cables need to travel around a bend, pulleys are often used to reduce friction. Unfortunately, space isn't always available for pulleys, such as in a minimally invasive flexible instrument.
As an example of such a flexible instrument, a robotically manipulated endoluminal device may be employed that enters the patient through a single minimally invasive incision or through a body orifice, such as the mouth, rectum, vagina, or urethra, to reach a surgical or diagnostic site within a patient by passing, at least partially along with way, through a natural body lumen. The endoluminal device in this case may integrate surgical instruments and an image capturing device into one unit.
One application for such an endoluminal device is Natural-Orifice Transluminal Endoscopic Surgery (“NOTES”), which may involve, for example, passing flexible instruments through one of the body's orifices and entering the abdomen from the inside of the patient, rather than through a minimally invasive incision from the outside. Among the many technological challenges in building medical robotic systems for NOTES, the medical devices used in such systems need to be long, slender, flexible, and steerable to maneuver around bends inside the lumen. Moreover, the medical devices must be articulate and yet provide sufficient force and accuracy to carry out necessary tasks at the distal end. Given anatomy size limitations, all actuators (e.g., motors) used to drive control elements/links (e.g., cables, rods, gears, etc.) to provide the medical device's steerability and articulation must be located at the proximal end of the medical devices which is generally outside the patient. This means that the actuators must be able to produce enough torque to overcome a substantial amount of friction created when the control elements slide over or otherwise interact with surfaces as they travel around numerous bends in their respective medical devices.
To avoid the friction problems associated with actuator elements such as described above, hydraulic or other servo mechanisms may be used to control the end effector at the distal end of the instrument in lieu of actuator elements. The hydraulic cylinder at the distal end can be bulky, however, compared to direct cable actuation. Further, the hydraulic cylinder and seal have friction of its own to contend with.
Alternatively, large actuators (e.g., in a hydraulic system) may be used to overcome the sliding friction. Such large actuators, however, present their own challenges in terms of lack of precise movements and the large space they require given the number of motors that may be needed to provide the required Degrees of Freedom (DOF) in NOTES medical devices. Accordingly, large actuators are not generally desirable.
“Dithering” is a commonly used method for compensating for stiction. Applied in this case, oscillating forces with a peak approximately equal to the stiction would be applied to the actuator elements so that the actuator elements are prevented from ever being fully at rest. Dithering, however, may result in vibrations that are uncomfortable to the operator of an instrument employing the actuator elements and result in wear and tear on mechanical parts. “Coulomb” friction compensation is a commonly used method for compensating for Coulomb friction. Applied in this case, the direction of the velocity of an actuator element is sensed and a compensating force is applied to the actuator element according to the sensed velocity direction in order to compensate for the Coulomb friction. Since it is difficult to measure velocity accurately when the actuator element is at rest, however, due to measurement inaccuracies, noise, and the like, it is problematic in applying the compensating force in the correct direction.